U.S. Pat. No. 3,766,976 discloses a heat exchanger for refrigerator evaporators in which tubing is wound in a circular or roughly elliptical helix and along which is formed an array of fins extending radially inward from the tube windings in the coil to enhance the transfer of heat of the tube coil with the air circulated across the coil. The coil is formed in a pair of sections, placed in side-by-side relationship in a compartment provided within the refrigerant cabinet, and air to be refrigerated is directed over the coils in a direction transverse to the axis of the coils. The air passes between the turns of the windings over some of the fins, such that the refrigerant circulated within the tubing absorbs heat and cools the air.
As discussed in detail in that patent, this heat exchanger configuration produces efficient heat transfer, while being relatively resistant to frost blockage and can be manufactured at low cost.
In addition the absence of external fins allows ready handling of the unit during assembly without likelihood of damaging the relatively fragile fins.
While evaporators of this type have come into use in recent years, the traditional heat exchanger for these applications comprised a plate-on-tube type heat exchanger in which loops of refrigerant-carrying tubing are joined by an array of parallel fins. The air to be refrigerated is passed over the tubing and between the fins to cause the refrigerant to absorb heat from the air and refrigerate the air. In this configuration, air flow is directed in a parallel direction to the tubing fins.
In both heat exchanger configurations, the fins are of course provided to enhance the transfer of heat between the air and the heat exchanger tubing carrying the refrigerant. The fins are of relatively low thermal mass and great surface area and are so able to rapidly absorb heat and conduct it into the tubing. In the plate type heat exchanger, the air may be directed in a parallel direction to the fins as noted and good air flow is afforded over the fin structure and is well distributed across the fin array. The plate-on-tube design, as noted in the specification of the above-referenced patent, has the advantage that the fins are relatively closely disposed and subject to ice bridging and are somewhat more costly to manufacture.
While the coiled tube evaporator is highly successful, several factors in regards to the mode of air contact with the fins could allow improved efficiency of the unit.
Firstly, the fins extend radially inward from the tube windings and the air flow being at a direction transverse to the coil axis, the leading section of each turn of the coil shadows a substantial proportion of the fins extending radially inward from the turn section. That is, it blocks the air flow from passing over the portions of the fins closely adjacent the section of the tube.
In the particular design described in the above-referenced patent, the fins extend radially inward, but there is a central opening between opposing fins in each turn to prevent total ice blockage. The opening is resistant to total bridging by ice build-up due to the relatively large size of the opening. However, this opening falls into the gap between windings such that the air tends to bypass the shadowed portions of the fins and pass through the central region intermediate the fin array to further reduce the degree of contact with the fins.
In addition, the air flow is more nearly parallel to the direction of the array of the fins since the tube windings are at a relatively steep angle with respect to the coil axis, which causes shadowing of the fins by other fins in the array.
The relatively steep angle of the tubing coil turns with respect to the coil axis for closely wound coils also creates gaps between the coil windings which, when viewed in the direction of air flow, are unoccupied by fins. This allows a substantial proportion of the air to pass over the coil windings without passing over any fins, to further degrade the efficiency of heat transfer.
While the obliqueness of the coil turns to the direction of the air flow could be increased by increasing the helix angle or the angle at which the tubing is wound into the helix, the helical angle is generally determined by the number of coil turns and the available space within which the coil is to be mounted.
It is accordingly an object of the present invention to provide an improved tube coil type heat exchanger in which the air flow contact with the radially extending fins in passing across the coils is increased.
It is yet another object of the present invention to provide such a coiled tube heat exchanger in which the proportion of the air flow passing between the gaps in the coil windings without contacting the fin array is reduced.
It is still another object of the present invention to provide such a coiled tube heat exchanger of improved efficiency without increasing substantially the manufacturing costs to preserve the advantages of the coiled tube configuration while improving its performance.
It is still a further object of the present invention to reduce the degree of shadowing by the leading portions of the tube windings and by the fins in the array which increases the proportion of air flow in contact with the fin array.
It is yet another object of the present invention to provide a coiled tube evaporator with an increased air flow contact with the fin array which does not substantially increase the space required to house the evaporator.